1. Field of the Invention
The present invention relates to LED heads.
2. Description of Related Art
FIG. 6 illustrates a general construction of a conventional LED type electrophotographic printer. Referring to FIG. 6, a photosensitive drum 11 rotates in a direction shown by arrow A. Disposed around the photosensitive drum 11 are a charging roller 12, an LED head 13, a developing roller 14, and a transfer roller 15. The charging roller 12 uniformly charges the surface of the photosensitive drum 11. The LED head 13 illuminates the charged surface of the photosensitive drum 11 to form an electrostatic latent image in the surface. The developing roller 14 applies toner 17 to the surface of the photosensitive drum 11 to develop the electrostatic latent image into a toner image. The transfer roller 15 transfers the toner image onto the print medium 16 passing between the photosensitive drum and the transfer roller 15.
When the LED head 13 illuminates the charged surface of the photosensitive drum 11, the light impinges the charged surface of the drum, so that, the charges in illuminated areas on the photosensitive drum 11 are neutralized. The developing roller 14 is negatively charged and applies negatively charged toner 17 to the neutralized areas on the photosensitive drum 11.
FIG. 7 illustrates a general construction of a conventional LED head 13. FIG. 8 illustrates the electrical wiring between an LED array 22 and an LED driver 26.
Referring to FIGS. 7 and 8, a plurality of LED arrays 22 are arranged side by side on an LED circuit board 21. The LED array 22 includes a plurality of LEDs 25 aligned in line on a p-type semiconductor substrate 29. The LEDs 25 are driven by the LED driver 26. Each LED array 22 is electrically connected with a corresponding LED driver 26 via wires 28 as shown in FIG. 8. A lens 23 is placed in the path of the light emitted from the LED array 22 so as to form an image of each LED on the surface of the photosensitive drum 11. A holder 24 supports the LED circuit board 21 and the lens 23 in position.
FIG. 9 illustrates a conventional LED array. The LED 25 includes a square light-emitting element 27 and the electrode 30 connected to the light emitting element 27. The light emitting element 27 is an n-type semiconductor region formed on a p-type semiconductor substrate 29 by diffusing an impurity. Each LED array 22 has 64 or 128 light emitting elements 27 at intervals of 300 to 600 dpi (dot per inch).
When the LED driver 26 drives the LED 25, the LED 25 emits light which is focused into a spot by the lens 23.
However, when a printing is performed at a higher resolution of about 1200 dpi, the aforementioned conventional LED head 13 cannot provide the same print quality as for a resolution of 300 to 600 dpi.
FIG. 10 illustrates a conventional LED array 22 used in a conventional LED type electrophotographic printer. Referring to FIG. 10, the light emitting elements 27 are aligned at intervals of q1, e.g., 21 microns corresponding to the resolution of the printer. The light emitting elements 27 have a width W1 of about 10 microns and are spaced apart by a distance u1 of about 11 microns. Too large a width W1 reduces separation of adjacent dots in the print and too narrow a width W1 results in poor connectivity of adjacent dots in the print. The optimum value of the width W1 is empirically determined.
If the LED arrays 22 are mounted on the LED circuit board 21 in such a way that the center-to-center distance between the last light emitting element 27b of one LED array 22 and the first light emitting element 27a of the next LED array 22 is q2, different from the interval q1 at which other light emitting elements are aligned on each LED array, all of the printed dots are not at the same intervals. Therefore, the LED arrays 22 are mounted on the LED circuit board 21 with the distance q2 adjusted equal to the interval q1.
In order that the LED arrays 22 are mounted on the LED circuit board 21 without touching each other, a maximum distance e1 from the endmost elements (elements 27a and 27b) to the edge of the LED array 22 must be 5 microns.
The LED array 22 is fabricated in semiconductor processes. Thus, the interval q1 depends on the accuracy of the mask, not shown, and may be fabricated with an accuracy of 1 micron. In contrast, the semiconductor wafer is mechanically diced into individual LED array 22. Thus, the dicing error ranges .+-.4 microns, i.e., 8 microns maximum.
Therefore, for example, if a nominal distance e.sub.nom from the last, light-emitting element 27b to the longitudinal edge of the LED array 22 is 5 microns, the distance e.sub.min becomes as short as 1 micron or less. With such a short distance e.sub.min, light emitted from the last light emitting element 27b of the LED array 22 leaks from the end of the LED arrays in the direction parallel to the surface of the LED array 22. As a result, the effective size of the last light emitting element 27b is larger than its physical size, causing larger dots than other spots. This impairs the print quality.